Droplets are encountered in nature and a wide range of science and engineering applications. Naturally occurring droplets are found in dew, fog, rainbows, clouds/cumuli, rains, waterfall mists, and ocean sprays. Showerheads, garden hoses, hair sprays, paint sprays, and many other commonly accepted devices are used to facilitate a dispersion of droplets into the surrounding air. Additionally, a variety of important industrial processes involve discrete droplets, such as spray combustion, spray drying, spray cooling, spray atomization, spray deposition, thermal spray, spray cleaning/surface treatment, spray inhalation, aerosol (mist) spray, crop spray, paint spray, etc. The related industrial areas span automotive, aerospace, metallurgy, materials, chemicals, pharmaceuticals, paper, food processing, agriculture, meteorology, power generation. Not withstanding the natural aspects of droplets, it is the increased desire for finer or smaller particles in industrial applications that led to the present invention's improvement in the atomization process. (Science and Engineering of Droplets by Huimin Liu.)
In addition to the general discussion of the state of the art presented herein, attention is also directed to Science and Engineering of Droplets, Fundamentals and Applications, by Huimin Liu (ISBN 0-8155-1436-0). In this book Ms. Liu presents a good overview of some of the various techniques currently used to atomize liquids.
At the present time, various atomization processes manufacture most metal powders. The principle underlying these processes is often the same: a liquid metal placed in a distributor is forced through a nozzle to obtain a thin jet which is dispersed in the form of particles by the rapid motion of a gas or of a stream of liquid.
Three classes of atomization processes can be distinguished. According to a first class, the liquid metal, in most cases, is atomized at the time of the casting. In a particular case of the process, the disintegration of the liquid into particles is produced by the mechanical action of a rotating disc, but, in general, the atomization is produced by air, gas, water, and under vacuum by bursting of the liquid due to a great pressure difference and dissolved gases coming out of liquid solution. An improvement to this scheme is pulsed plasma atomization where a plasma shock tube is used to impart very high impulse loads on the descending melt leading to finer particles. (U.S. Pat. No. 5,935,461) Another recent development is to force molten material through small holes as in Pulsed Atomization. (U.S. Pat. No. 5,609,919) Spraying of solid particles has also been mentioned, but so far has been limited to the agglomeration of, or the introduction with, the dispersible liquid material.
Another class of processes has been developed a little more recently. This is atomization by centrifugal force which is applied according to two variants: either the melting electrode forms the starting material for obtaining the particles, or the distributor containing the liquid is subjected to a rotation which causes the ejection of the liquid in the form of drops against the cooled walls of a plant, thus enabling a powder to be recovered. In each of these cases atomization occurs when the centrifugal force of the particle exceeds the surface tension retaining force.
Finally, a last class consists of processes employing ultrasonic technology, a vibrating electrode, and cooled rolls that rotate. (U.S. Pat. No. 5,876,794)
There are some other “laboratory stage” methods of atomization. Papers have been presented (2002 World Congress on Powder Metallurgy and Particulate Materials June 2002) that included descriptions of Impulse Atomization, and Plasma Atomization. Exploding wire atomization is in commercial use at Argonide Nanomaterials Corp. Flame synthesis is used commercially by AP Materials (U.S. Pat. No. 5,498,446).
Impulse atomization is a technique where the melt is forced through holes in ceramic material. The size of the resulting powder is proportional to the size of the holes. It is believed that the smallest powders this technique could ever produce would be approximately 20 μm. Plasma atomization is a simple process where a sacrificial wire is subjected to the blast of a plasma jet (U.S. Pat. No. 5,707,419). This high temperature blast is strips off molten material that becomes powder.
There are also four patents and one published patent application that relate to this area of endeavor that may warrant attention relative to the present invention. While only the first is strictly an atomizer i.e. the material is melted, converted to smaller units then these smaller units are solidified, all relate to the manufacture of fine metal powders. The first (U.S. Pat. No. 5,935,461) outlines a technique where a pulsed plasma jet is used to blast a stream of molten material in a manner similar to gas atomization.
The next three involve techniques where the material(s) to be subdivided into particles are vaporized then condensed. The second, (U.S. Pat. No. 5,788,738) is such a device. The third, (U.S. Pat. No. 5,514,349) is a variation on that approach. The fourth, (U.S. Pat. No. 6,580,051) uses an electro thermal gun to improve the exploding wire technique. Lastly, U.S. patent application US20030126948A1 discloses a means of producing high purity fine metals, metal oxides, nitrides, borides, carbides and carbonitride fine powders using a high temperature chemical reaction/precipitation technique.
There are other methods of producing metal powders that use centripetal acceleration to enhance the process. These methods are outlined in Powder Metallurgy Science, German (ISBN 1-878954-42-3). The disk and cup methods require the liquid to be forced radially outward thus thinning the melt prior to release and atomization. The mesh and rotating electrode methods use centripetal acceleration to pull drops away from the parent material. Dr. Yunzhong Liu—National Institute for Materials Science (Japan) presented a paper at the 2002 World Congress on Powder Metallurgy & Particulate Materials conference where he described a hybrid gas and centrifugal atomization system.
The means to manufacture fine metal powders can be broken into two broad categories. First there are those methods that vaporize the material or some compound of the material then precipitate the material out of the vapor or gaseous form through either a chemical reaction or heat removal.
Those techniques of the second means spread a molten material into thin liquid layer until instabilities force the layer to disintegrate into smaller units. Due to surface tension these units quickly form spheres. Heat is removed resulting in powder. The invention we're attempting to protect falls into this second category.
Before the technical discussion of the present invention commences, it may be valuable to specifically identify at least one of the particular industrial applications that will be significantly benefited by the development of the present invention. Metal Injection Molding (MIM) is a manufacturing technique where a slurry of fine powdered metal and binder are forced into a metal cavity in a manner very similar to plastic injection molding. The slurry hardens in the mold and the hardened material (called a compact) is released. The binding agent is then removed from the metal by one of several different means. The remaining metal is placed in a furnace and sintered.
During sintering the compact shrinks as the individual powder particles join to one another ultimately reaching full density. The industry standard is to use powder of approximately 15 μm diameter for this application. This process can be improved by using smaller diameter particles. Smaller particles sinter more readily, which would enable the duration and/or the sintering temperature to be reduced. Smaller particles also reduce the surface roughness of the finished part.
The current commercial techniques for atomizing metals i.e. gas, water and centrifugal atomization, are, for the most part, mature technologies that are impractical techniques to produce the still smaller powders and particles needed to advance the industry. Something new is needed.
Diminishing the size of atomized metal powder serves two purposes: it permits more rapid and/or lower temperature sintering and it allows heat to be extracted from the atomized material more rapidly. These two effects are interrelated.
While the increased surface energy inherent to a smaller particle is not a trivial contribution to technology, the large contribution this invention offers is the ability to cool the particles quickly. High cooling rates lead to reduced particulate microstructure grain size and in extreme situations amorphous microstructures. Rapidly solidified (small grain size) alloys can lead to improved magnetic, electrical, mechanical, wear and corrosion properties (Powder Metallurgy Science—German ISBN 1-878954-42-3). Smaller crystalline grains lead to a greater portion of the solidified material being grain boundaries that enables elevated diffusion during sintering. Operationally, the elevated diffusion allows decreased sintering temperature and/or duration.
While the known atomization processes of the state of the art exhibit features that are not insignificant, such as, obtaining very dense and homogeneous particles with a good purity and an efficient control of the composition, in most cases, they cannot make very small particles, are uneconomical in doing so, or are incapable of making alloys.
The present invention overcomes the shortcomings of the existing technologies by introducing a novel and non-obvious process for manufacturing particles that are significantly smaller (finer) and cooled more quickly than currently possible through known atomization techniques. Without question, the availability of smaller finer particles through the atomization techniques of the present invention will allow noteworthy advancements in a variety of manufacturing environments, such as in MIM.
As stated earlier, the present invention relates to a novel process for atomizing a dispersible liquid material or a mixture of dispersible liquid materials. More specifically, the present invention utilizes bursting bubbles, surface waves, and splashes to create fine particles by purposely introducing gas flow on the liquid material(s) to be atomized while these material(s) are simultaneously at an elevated acceleration: thereby significantly enhancing the physical characteristics of the resulting particles, i.e. miniaturize, while reducing contamination threats by avoiding physical contact between the material(s) being atomized and any refractive materials. In other words, the present invention advances the art by utilizing the inertial forces of an elevated acceleration environment to miniaturize the process of atomization seen in nature.